1. Field of the Invention
The invention relates to a private branch exchange apparatus which encloses therein a plurality of external lines and a plurality of extensions.
2. Related Background Art
Hitherto, various kinds of telephone exchange apparatuses such as private branch exchange (PBX), main apparatus in a key telephone apparatus, and the like have been provided. In such a kind of apparatus, a connection control at the time of reception or transmission between a plurality of external lines and extension telephones or the like is executed. However, in the case where a new call is received from the external line, the telephone of the extension is rung, thereby informing the reception.
In the above conventional apparatus, however, since all of the receptions are made effective, for example, in the case where the number of persons who pick up the handset is very small because of the going-out or the like, there is a problem such that it is impossible to respond to the reception and the partner who dialed the telephone listens to a ring back tone for a long time, so that he will consider that nobody is there.
In the case where the conventional private branch exchange is congested and all of the external lines are busy because they are in use, even when an emergency situation occurs, it is impossible to transmit from the external line to a police station (dial number is set to 110), a fire station, emergency medical situation (dial number 119), or the like, so that the user must wait until a free external line is found out.
FIG. 4 is a block diagram of a private branch exchange for explaining a transmitting procedure in the conventional private branch exchange (PBS). FIG. 5 is a flowchart for explaining a flow of the transmission procedure.
In FIG. 4, an extension terminal (TEL) 702 is connected to an extension of a private branch exchange (PBX) 701. Analog lines 705 and 706 from an analog public network 703 and ISDN lines 707 and 708 from an ISDN network 704 are connected to the external line.
The PBX 701 comprises: a TEL extension interface device (TELI) 709 to interface with the TEL 702; an analog line interface device (COT1) 710 to interface with the analog public network 703 as a connecting section with the analog line 705; an analog line interface device (COT2) 711 to interface with the analog public network 703 as a connecting section with the analog line 706; an ISDN line interface device (BRT1) 712 to interface with the ISDN network 704 as a connecting section with the ISDN line 707; an ISDN line interface device (BRT2) 713 to interface with the ISDN network 704 as a connecting section with the ISDN line 708; a busy tone sound source (BT) 714; a speech communication path switch (TSW) 715 to connect an audio channel; a main memory device (MM) 716 having an external call state storage table to store a call state of the external line; and a control circuit (CC) 717 for controlling the whole PBX 701 and executing a reception control and a transmission control.
In the above system, it is now assumed that an emergency situation occurs and the partner's telephone number "0110" is dialed and a call is transmitted in order to inform a warning to the police from the TEL 702 of the extension (step S401 in FIG. 5).
The first number "0" of the dial No. "0110" indicates the external line transmission special number and is the number which is added when transmitting from the extension to the external line. The remaining numbers "110" indicate the telephone number of the police. When there is a transmission from the TEL 702, the TELI 709 detects the transmission and informs the transmission to the CC 717. The subsequent procedure slightly differs in dependence on whether the TEL 702 is a terminal to transmit by sending the dial numerals one digit by one like an analog single telephone (divisional transmission) or a terminal which transmits by sending all of the dial numerals in a lump like an ISDN terminal (batch transmission).
The case where the TEL 702 is a terminal which performs the divisional transmission will now be described. When the TEL 702 is a terminal which performs the divisional transmission, the CC 717 knows that the transmission is an external line transmission at the state of the reception of the first numeral "0" and checks to see if there is a free external line or not with reference to the external line call state storage table in the MM 716 in order to decide the external line which transmits (S402, S403). The CC 717 takes either one of the following procedures (1) and (2) in accordance with the state of the external line call state storage table.
(1) When there is a free external line (S404), one of the external lines which transmits is selected from among the free external lines and the selected external line is captured and the external line transmission is executed (S405).
(2) When there is no free external line (S404), the TSW 715 is controlled, the TEL 702 and BT 714 of the extension which transmitted are connected, the TEL 702 is allowed to listen to a busy tone, and a fact that the transmission was refused because all of the external lines are busy is notified (S406).
The case where the TEL 702 is a terminal which executes the batch transmission will now be described. When the TEL 702 is a terminal which executes the batch transmission, the CC 717 analyzes the dial numerals and knows that the transmission is an external line transmission with reference to the first numeral "0" and checks to see if there is a free external line or not with reference to the external line call state storage table in the MM 716 in order to determine the external line which transmits (S402, S403). The CC 717 takes either one of the following procedures (1) and (2) in accordance with the state of the external line call state storage table.
(1) When there is a free external line (S404), one of the external lines which transmits is selected from among the free external lines and the selected external line is captured and the external line transmission is executed (S405).
(2) When there is no free external line (S404), a fact that all of the external lines are busy and the transmission was refused is informed to the TEL 702 (S406).
FIG. 6 is a diagram showing a structure of the external line call state storage table. It is now assumed that there is no free external line and the transmission was refused to the TEL 702.
When the transmission is refused, the TEL 702 once releases the call. However, since an emergency situation occurs, the partner's number "0110" is redialed and the transmission is performed (S407). The PBX 701 refuses the transmission until a free external line is found out. The TEL 702 continues the transmission until the external line transmission can be executed.
When the call of the external line is soon released and the free external line is found out, the CC 717 rewrites the value of the external line call state storage table to the free state from the in-use state. It is now assumed that a free line occurs in the B1 channel of the ISDN line 708.
When the call of the ISDN line 708 is released, the BRT 713 detects the call release and notifies to the CC 717. When the release notification is received, the CC 717 rewrites the value corresponding to the B1 channel of the ISDN line 708 in the external line call state storage table to the free state from the in-use state and controls the BRT 713 and executes the call releasing process.
It is assumed that the TEL 702 dials the partner's number "0110" and transmits just after the call release. When the TEL 702 transmits, the TELI 709 detects the transmission and notifies the transmission to the CC 717. When the transmission notification is received, the CC 717 analyzes the partner's number and knows that the transmission is the external line transmission.
The CC 717 subsequently searches the free external line with reference to the external line call state storage table in the MM 716 in order to decide the external line which transmits. When the CC 717 knows that there is a free line in the B1 channel of the ISDN line 708, the B channel is captured and the value corresponding to the B1 channel of the ISDN line 708 in the external line call state storage table is rewritten to the in-use state from the free state. The B channel is captured, the BRT 713 corresponding to the ISDN line 708 is controlled, and the transmission is performed to the ISDN network 704. The partner's number (reception number) in this instance is equal to "110".
After that, when the partner responds, the CC 717 controls the TSW 715 and connects the TEL 702 of the extension and the ISDN line 708 of the external line. When the TSW 715 is connected, a speech communication between the TEL 702 and the ISDN line 708 is started.
However, in the above conventional apparatus, in the case where an emergency situation occurs and the external line transmission for emergency is executed from the extension terminal, so long as all of the external lines are in use, the external line transmission cannot be immediately executed and the user of the extension terminal must transmit many times until a free external line is found out, so that it is very inconvenient.